1. Field of the Invention
The invention pertains to the fields of agriculture and energy production. More particularly, the invention pertains to systems, methods, and structures for the production of biogas for heat and energy recovery from on-farm animal and plant residues.
2. Description of Related Art
Biogas production is a common feature on many farms worldwide, whereby the methane content is employed as the source of combustible energy. Farmers may customarily produce methane by exclusion of air inside sealed containers to produce a mixture of fermented gaseous byproducts as biogas. It has become very common to construct liquid-based manure systems in which outflow of daily farming operations, including manure wash and barn milk parlor, washes together with miscellaneous farm residues such as hay, straw and spent silage and feed. These mixtures are pushed or gravity-fed into cement in-ground tanks, in which, by applying elevated temperature and swirling action, rapid methanogenesis occurs by nature of the bacteria in the mixture indigenous to the gut of ruminants, especially cows. The net result is the production of gas, in particular methane-enriched gas.
It is possible within a dairying operation to run a digester with no requirement for any outside inputs, save only the manure in liquid form. Some dairies supplement digesters by adding fermented milk solids waste, yogurt waste, whey from cheese-making and the like, even to the extent of adding pulped or slurried food waste from area sources. In virtually all of these cases, the normal function of the digester is maintained so long as the flow characteristics of the influents and effluents are held within normal boundaries, as defined by pipe and trough volume and flow restraints. If the material becomes too thick, as from heat evaporation of manure in summer while on the floors or from inclusion of wastes that have elevated solids content, then wash water, water, or similar very low solids liquid is added to maintain consistency. The result of the liquid-based approach on farms is massive lined-steel or concrete tanks requiring a sizable capital input.
Methane is the common desired product of anaerobic fermentation of organic residues, whereby both carbon dioxide (CO2) and methane (CH4) are formed, together called biogas. Other smaller components of biogas are water (H2O) and hydrogen sulfide (H2S). Inoculums other than manure are uncommon. Manure that is allowed to stand outdoors in a pile may have a reduced methane content as a result of poisoning of methanogens by the presence of regular air, in particular by the presence of gaseous oxygen.
The methane process requires a certain period of time to go through stages of anaerobic decomposition to achieve the resulting methane-enriched gas. This time period may vary in terms of mean residence times in the range of 5 days to 30 days. While ruminant manure is ideally predisposed for methane formation, the addition of quantities of other waste, including straw, silage, and food scraps, necessitates a pre-fermentation for the breakdown into component fatty acids, especially acetic acid, a direct precursor of methane gas. This may constitute a lag phase of one to several days, and if the feed stream is loaded improperly or excessively with outside ingredients, may dispose the reactor to sourness or pH crashes, recovery from which certainly meaning lost time in digestion. Maintaining swirling or moving action in order to maintain contact between freshly input materials and those already fermented in the tank is very important to provide a buffering action that helps eliminate swings in fermentation performance and behavior. On some occasions buffering agents, such as bicarbonate, are added to input materials, such as food scraps, to avoid excessive punch down in biological activity in the tank.
An additional and significant problem with liquid-based systems, in addition to requiring massively sized tanks, is that the entire farm must essentially convert to liquid-based materials and flows and must maintain a farm disposal system for the liquid effluent resulting from the digestion process. Effluent production is a daily phenomenon of a liquid system, and storage space for liquid effluent is limited. Storage of effluent, in fact, typically requires more tanks and holding capacity, and while not strictly aerobic after coming out of the digester, the open tanks continue to emit background or fugitive methane, which is a significant atmospheric global deterrent and often cited as a negative carbon balance for dairying. Rapid incorporation, therefore, of liquid effluent into surface soils, which by nature of being very aerobic, reduces any further methane output to a minimum and is the only practical solution to the problem. Such action is not a very feasible option to farmers during the winter and summer seasons because of limited access to useable surface soil together with environmental regulatory restrictions. In winter in colder climates, frozen ground greatly limits or prevents uptake of the effluent. During the growing season, the presence of agricultural crops and pasturing preclude addition of fresh waste liquids to many areas. Excess storage of liquid effluent, in addition to influent manure, creates the potential for overflow and entrance to surface and groundwater, particularly in inclement seasons with poor weather and above-average rainfall. The pernicious cycle of open-air liquid systems fostering liquid restraints, sensitivity to rainfall excess that causes overloading of lagoons, and environmental damage from leaked fugitive gases and nutrient contamination of waterways from these liquids poses real constraints to the liquid-based theory of methane digestion.
Construction of large, in-ground, heated concrete tanks to handle the liquid effluents is a real cost constraint for dairy farms, and it has been proposed by many that liquid methane systems are not viable for farms with under 500 head of cattle. It is, however, well known that methane biology is extremely scalable, meaning that the fermentation process is not, in fact, limited by size and there is no efficiency of scale pertinent to the biology. Medium or smaller farms number in the thousands in America, yet methane digestion technologies, by the nature of focusing on a minority of large to very large dairy farms, such as 2,000 to 5,000 head operations, have thereby effectively excluded provisions for medium and small farmers of cost-effective environmental solutions to manure and waste handling, and therefore have hampered America's ability to expand energy-efficient futures.
Most farms would prefer to have an opportunity to better handle waste, and especially to produce energy if cost effective, as farms require a constant flow year round of both fuel and electricity. Biogas, once produced, may be used as unfiltered, uncompressed gas for boilers and heating and may also be fed to engines for direct conversion to electricity, or the combined heat and power (CHP) units to produce heat and electricity simultaneously. Also, biogas, once cleansed of excess water, hydrogen sulfide and carbon dioxide, may be prepared as compressed natural gas (CNG, also referred to as compressed biogas or CBG in Europe) and used to co-fuel farm diesel tractors and other vehicles, as a primary fuel source, a conversion that is increasingly practiced in the world. Since the majority of the world's farms are medium to small scale, the absence of viable methane technologies for small scale approaches means that common solutions to increase viability and sustainability of farm operations from an energetic and cost-effective point of view are lacking.
While many very small farms in third-world regions have addressed the situation by design and construction of small to very small digester systems run almost entirely by manual farm hand labor, small farms in industrialized nations do not have such hand labor available and have not chosen to build these very flexible, small-scale systems. There is an evident gap in the farm energy methane digester capable of providing for a family farm of one to four cows or llamas or 10-30 sows, such as are common in developing nations and third world regions, and those small farms in the United States, which may have in the range of 75-300 cows, in other words large by third-world standards, but in fact small by western-world modern standards. It is this gap that the current systems, methods, and structures propose to fill.
Most of the commercial industrial engineering firms that design and construct farm methane digesters are trapped in this costly and somewhat environmentally unfriendly struggle to handle on-farm liquid wastes and provision of electricity and heat energy to large and super-sized farms, where undoubtedly the biogas solution is better than no action at all. It does not solve the liquid handling problems and it entails huge capital reserves and debt to cover costs. Additionally, in many regions the concept of public purchase of excess electricity generated by these large 100-500 kW systems is not sufficiently developed other than an offer of net metering to farm owned facilities or recapture by utility companies at grossly devalued worth, meaning the energy creation from methane is not a very viable path. The efficiency of gas-to-electricity conversion, being as low as 35%, and the cost-effectiveness of such electricity generation in view of sometimes poor returns on sold or net-metered electricity, in addition to requiring a significant capital investment in energy generation units, also restricts the usefulness of the electricity approach. Creating a cost-effective methane system, however, within a small farm environment, where a focus on heat production from biogas, which is much more efficient at 70-90% efficiency, and cleansing and compressing for vehicular on-farm use as transport fuel, may make it more sustainable than larger systems that convert the biogas to electricity.
The liability for over-sized, over-designed farm energy production operations falls almost entirely on the farmers themselves, as engineers and investors rarely assume management positions in on-farm systems, and failure therefore is the sole risk of the farmers as well, this coming at a time when on-farm debt has grown disproportionately to farm sustainability. This is very costly to society as a whole, since failure of farms inevitably drives up costs of food and results in more environmental destruction from larger and super-sized farming operations.